1. Field of the Invention
The invention relates to surface coating solutions and films, and particularly to coatings having anti-reflective and anti-static properties.
2. Related Art
In optical applications, substrates such as glass and plastic have been widely used. Plastics, as compared with glass, are less expensive, more easily formed into complex shapes, lighter in weight and generally not as brittle. Accordingly, the use of plastics, such as polycarbonates and acrylic polymers, is increasing in optical applications. Both glasses and plastics, however, suffer from reflective losses at the substrate/air interface, the losses averaging about seven percent of the transmitted light for the two surfaces. The reflective loss is even more severe in substrates having high index of refractions (e.g., index of refractions on the order of 1.55 or more). While numerous practical approaches to reducing the reflective losses for glass substrates have been developed, few low temperature techniques are available for plastic substrates at low cost.
Anti-reflective (AR) coatings or films offer one way to reduce the reflective losses at the substrate/air interface. AR coatings reduce the reflectance of light from a surface thereby increasing the light transmittance through the coating/substrate interface. One challenge to using AR coatings on organic substrates like plastic is forming such films at temperatures that will not melt or deform the substrate.
Low temperature AR coatings are primarily made by vacuum deposition techniques. In general, there are two types of vacuum deposition techniques: vacuum evaporation and vacuum sputtering. In vacuum evaporation, a charge of the material to be evaporated is placed in a crucible of a refractory material. An electrical resistance heater heats this charge (either by conduction or by radiation) after the chamber has been evacuated to about 10xe2x88x926 torr. As the temperature of the charge rises, its vapor pressure rises and a significant evaporation develops. The evaporant then condenses on the cooler substrate. Vacuum sputtering, on the other hand, is characterized by a momentum transfer process in which argon or other ions and atoms from a plasma bombard a target made of the material to be coated. The collision of the argon atoms and ions with the surface atoms and molecules of the target knocks off (sputters) the target material, which then forms a deposit on the substrate. Both of the vacuum deposition techniques (evaporation and sputtering) produce high quality AR coatings or films. Generally, the disadvantages of vacuum deposition are: (1) large capital expenditure for deposition equipment, (2) possible temperature build up that can deform or melt a plastic substrate, (3) restricted substrate size and geometry due to equipment limitations, and (4) separate coating processes may be required for each surface.
U.S. Pat. No. 5,719,705 discloses a transparent multi-layer AR coating wherein each layer comprises an electrically conductive, high-refractive index or an electrically-conductive, low-refractive index material using vacuum deposition techniques: electron beam reactive evaporation, ion-assisted deposition, and reactive sputtering of metal targets. The resultant AR coating, in addition to not attracting dust and other airborne contaminants, and has hydrophobic properties.
In addition to vacuum deposition techniques, AR surfaces can also be formed by chemical modification of a surface by a reactive plasma at low temperature. Again, this generally requires expensive equipment, possible heat build up, and size limitations.
Solutions containing fluorinated organics have also been deposited on plastic substrates which then exhibited AR properties, such as an apparatus described in U.S. Pat. No. 5,198,267. Fluorination processes appear, however, to be limited to self-developing photographic film applications and do not appear to be adaptable for large scale applications.
U.S. Pat. No. 4,966,812 describes the development of a process for applying a single layer, AR coating to a plastic substrate via micro-structural tailoring of a sol-gel solution. In the process, a silicon alkoxide, metal alkoxide or a mixture thereof is subjected to hydrolyzation then condensation in a solution to form a sol-gel hydrolyzation, followed by further condensation to form a polymeric reaction product. The gel formed from the sol- is reliquified. The reliquified gel is then diluted to increase its stability and to form the sol-gel AR surface coating solution. From this solution, a film with a low index of refraction (e.g., on the order of 1.22) can be deposited on a plastic substrate without heating or etching. This reliquified gel contains large polymers in solution resulting in a deposited film with greater porosity, and hence a lower refractive index. By careful control of the coating rate, the optimum film thickness can be obtained.
U.S. Pat. No. 5,476,717 describes a process to make a single layer AR coating for a plastic substrate. This single AR coating layer is formed from colloids of silica in a siloxane binder. Several other layers have been integrated into the process in order to enhance the adhesion, abrasion resistance, and hydrophobic properties. In general, the structure comprises an organic or inorganic substrate, successively covered by an adhesion-promoting coating made from a material chosen from among silanes, an anti-reflection coating of silicon colloidals coated with a siloxane binder, a coupling agent coating formed from a material chosen from among the silaxanes, and an anti-abrasive coating of a fluorine polymer.
U.S. Pat. No. 5,580,819 describes a composition for producing durable coatings and a process for preparing a single-layer AR coating on solid substrates, such as glass, ceramics, metals, and organic polymeric materials. The coating composition comprises, in combination, acid-catalyzed hydrolysis and condensation products of a water-silane monomer mixture and a film forming amount of a polymer having functional groups selected from amino, hydroxy and carboxy, hydroxy and amino, amino and carboxy, and amino, hydroxy and carboxy. The described process comprises applying the aforesaid coating composition (or an acid catalyzed sol-gel coating composition), substantially free of pre-formed oxide sol and water soluble metal salt, to the surface of a solid substrate, curing the applied coating, and treating the cured coating with an aqueous electrolytic solution for a time sufficient to produce a coating having graded porosity which is anti-reflective over a broad band of the visible spectrum.
U.S. Pat. No. 4,361,598 describes the use of sol-gel techniques to deposit two layer AR SiO2/TiO2 coatings onto solar cells and stainless steel or silicon ribbon. The refractive index range attainable using mixtures of these solutions is 1.4-2.4. The refractive index required for AR film on plastics is generally about 1.22 and cannot be achieved using the method described without the introduction of porosity. In addition, the method described requires heat treatments considerably higher than the typical upper temperature limits of plastics. Refractive index control is achieved by compositional control, firing temperature (300xc2x0 C.-600xc2x0 C.) and the firing atmosphere.
U.S. Pat. No. 5,858,526 primarily describes a method to make a two-layer AR coating having a high reflective index by a sol-gel solution process. Metal oxide colloids are coated with a polyvinyl material and rendered soluble in water-containing molecular solvents. The coating consists of a half wave-thick zirconia-polyvinyl pyrolidone layer of 1.72 refractive index and a quarter wave-thick porous silica-siloxane layer of 1.26 index. Both layers are centered at 600-nm wavelength. In general, this two layer AR coating is not abrasion resistant, and the AR coating also has poor adhesion. To achieve a moderate abrasion-resistance and hydrophobic behavior, the coating is overcoated with a very thin layer of a lubricating material. This lubricating material slightly permeates the porous silica layer underneath and hence increases the refractive index value a bit from 1.26 up to 1.30. In addition, several adhesion promoting layers containing organosilanes generally must be deposited between the layers to promote adhesion.
Coating compositions having anti-reflective and anti-static properties, a coating derived therefrom, a substrate coated with the coating and methods for their preparation are described. In one aspect, a coating in accordance with the invention comprises a multilayer or composite layer of a sol-gel alkoxide polymeric layer and a conductive colloidal metal oxide layer over a hardcoat or scratch-resistant layer on a substrate.
Objects, advantages, and novel features of this invention will be apparent to those skilled in the art upon examination of the following detailed description and accompanying drawings learned in the practice of the invention.